There are known to the art a number of aluminum products which contain polynucleate aluminum ions in solution. These products have been produced in response to the demand for more effective chemicals for use in water purification, paper sizing and plant dewatering processes. The products exhibit considerably improved properties for use within these technical fields, due to the higher electrical charge of the polynucleate metal ions in comparison with the earlier simple compounds.
The aluminum products that have been developed are principally of two kinds: chloride-based basic aluminum compounds, and sulfate-based basic aluminum compounds. With regard to the first group, there was initially developed a polyaluminum chloride (PAC) of the general formula EQU [AlCl.sub.x (OH).sub.3-x ].sub.n
where x is &lt;3, normally 1-2. Such compounds and methods for their manufacture are described, for example, in SE-B-7201333-7, SE-B-7405237-4, SE-B-7412965-1, SE-B-7503641-8, and DE-A-2630768.
The other type of aluminum chloride solutions (PALC) which are also based on the same polynucleate complexes, have general formulas which can be written as: EQU [AlClhd 3.multidot.x AOH ].sub.n
where A signifies an alkali metal, and EQU [AlCl.sub.3 .multidot.(x/2B(OH).sub.2 ].sub.n
where B signifies an alkali earth metal, n is a positive integer, and x is a number in the range 1 to 2.7. Polynucleate aluminum chloride solutions of the PALC-type are described in FR-Al-7512975, according to which the solutions are prepared by alkalizing aluminum chloride solutions with solutions of alkali hydroxide. According to this reference, however, it has not been possible to produce clear, stable solutions other than in a highly diluted state.
The term "stable solution" means a solution which will not change significantly with regard to its composition and properties, even when stored for long periods of time. It is stated in the FR-Al-7512975 that a solution containing up to 0.40 moles of aluminum per liter can be obtained under certain circumstances. The stability of the solution, however, is greatly limited and the solution must be injected directly into the water to be treated. It is clearly evident from the reference, and in particular from the working examples, that the known PALC solutions which have aluminum concentrations above about 0.1 mole/liter cannot be expected to be effective and stable.
Sulfate-based basic aluminum compounds have been described in EP-A-79850039-3, EP-A-80850033-4 and SEA-8101830-1. These products contain polynucleate metal ions in solution, to a greater or lesser extent, and are thus effective water cleansing agents. The sulfate-based products can also be used for purposes other than for cleansing water in which the presence of polynucleate metal ions favors the effects desired. In certain cases, however, it is highly desirable, and even necessary, to restrict the supply of sulfate ions to the smallest possible amount.
Low sulfate is particularly important when using the product to produce drinking or tap water. Sulfate reduction is also important in the case of systems which are used and cleansed repeatedly in order to eliminate the risk of sulfate accumulations in tissue and in water. This applies to water purifying systems used in areas where there is a water deficiency, necessitating the repeated use of available water for as long as possible, with intermediate cleansing of the water. In water such as this, after cleansing the water ten times with conventional aluminum sulfate, or compositions having corresponding sulfate contents, the sulfate content of the water may reach such high levels as to result in a corrosive attack on the water-carrying conduit systems, resulting in troublesome leakage. The problem of enrichment of sulfate-ions has now also become manifest in the manufacture of paper, where the water transportation system is, to a large extent, completely closed for environmental purposes. The manufacturing processes are seriously affected by excessively high salt concentrations in the paper stock.
U.S. Pat. No. 4,238,347 discloses a method of producing sulfate-lean basic aluminum sulfate of the formula: EQU Al(OH).sub.x (SO.sub.4).sub.y (H.sub.2 PO.sub.4).sub.z (H.sub.2 O).sub.w
where
x is 0.75 to 1.5; PA1 y is 0.7 to 1.07; PA1 z is 0 to 0.2 and PA1 w is 2.0 to 4.2, wherein PA1 x+2y+z is equal to 3. PA1 n is an integer; PA1 x is 0.75-2.0; PA1 y is 0.5-1.12; PA1 x +2y is 3; PA1 z is 1.5-4 when the product is in solid form, and PA1 z is &gt;&gt;4 when the product is in solution, PA1 n is an integer; PA1 M is a trivalent metal ion PA1 x is 0.75-2.0; PA1 y is 0.5-1.12; PA1 x+2y is 3; PA1 z is 1.5-4 when the product is in a solid form and PA1 z is &gt;&gt;4 when the product has the form of an aqueous solution, and PA1 calcium sulfate hemihydrate. PA1 M is a trivalent metal ion; PA1 n is an integer; PA1 x is 0.75 to 2.0; PA1 y is 0.5 to 1.12; and PA1 z is 1.5 to 4. PA1 x is 1.1-18.; PA1 y is 0.6 to 0.7; and PA1 z is 2.0 to 2.5.
The compounds are prepared by reacting aluminum sulfate with the comminuted calcium carbonate, optionally in the presence of phosphoric acid, and passing the reaction mixture through a filter to isolate the gypsum formed. It has been found, however, that the carbon dioxide leaving the reaction process from the calcium carbonate, together with the gypsum create certain technical problems.
SE-A-8104149-3 describes an improved method for producing sulfate-lean polynucleate aluminum hydroxide complexes of the formula: EQU [Al (OH).sub.x (SO.sub.4).sub.y (H.sub.2 O).sub.z ].sub.n
where
wherein aluminum sulfate is reacted with one or more compounds taken from the group CaO, Ca(OH).sub.2, BaO, Ba(OH).sub.2, SrO, Sr(OH).sub.2 in aqueous solution to form the aforesaid compound, whereafter the resultant alkali earth metal sulfate precipitate is isolated and the residual solution optionally evaporated. The resultant solution has excellent properties and can be obtained with a high basicity, OH/Al.ltoreq.2.0. Manufacture is complicated, however, since isolation of the alkali earth metal sulfate is difficult to achieve and places specific demands on the technical equipment used.
Still a further method of producing these complexes is described in SE-A-8206207-6, in which a solution of an aluminum salt is neutralized to pH 5-7 to precipitate amorphous aluminum hydroxide, which is isolated and thereafter contacted with sulfate-ions in the form of aluminum sulfate and/or sulfuric acid to a "y" value of 0.5-1.12, preferably 0.5-0.75, this product being optionally converted to a solid form.
Another method of producing polynucleate aluminum hydroxide sulfate complexes is described in SE-A-8206206-8. This method comprising cooling a solution containing a polynucleate hydroxide sulfate complex of the formula: EQU Al.sub.m (OH).sub.n.sup.(3m-n)+
in which the anion is (SO.sub.4) .sub.(3m-n)-(3m-n)/2 where m and n are integers. These materials are prepared by alkalizing aluminum sulfate with NaOH or Na.sub.2 CO.sub.3, so as to crystallize out Na.sub.2 SO.sub.4 .multidot.10H.sub.2 O which is then separated out and the residual solution optionally concentrated or dried to solid form by evaporation.
The conversion of solutions of polynucleate aluminum complexes to a solid form in order to maintain the stability of the complex in those instances when the presence of costly stabilizing agents is not desired, is both complicated and expensive, such that the final product is not truly competitive with standard aluminum sulfate (BOLIDEN.RTM. ALG) or aluminum-ferrisulfate (BOLIDEN.RTM. AVR), for use in water cleansing processes. Water cleansing processes, drinking water, and domestic sewage purification processes require the presence of large quantities of flocculating agents for precipitating out the impurities present, such as, inter alia, phosphorous, and the cost of the water cleansing chemicals required in sewage purification in particular is very high. Demands have therefore been made for more effective and relatively inexpensive chemicals.